The present invention is generally directed to an apparatus and method for biaxially stretching extensible polymeric films and nonwoven webs in the cross-machine and machine directions, and the material produced therefrom. More particularly, the present invention relates to an apparatus and methods for biaxially stretching films and nonwoven webs in the cross-machine and machine directions while reducing the levels of variability in stretch.
Polymeric nonwoven webs, films and laminates made therefrom exhibit properties which make them especially suitable for use in personal care products. For example, such materials may be used as outer covers for personal care products such as diapers, training pants, incontinence garments, and feminine hygiene products. Additionally, such materials are particularly suited for use in protective outer wear such as coveralls, surgical garments, and face masks.
In particular, film laminates have become an important article in commerce, finding a wide variety of uses. Such laminates have been used in personal care products to create soft, barriers that provide stretch. The ability to xe2x80x9cgive without rupturexe2x80x9d and in some instances xe2x80x9cstretch,xe2x80x9d allows for body conformance by these products.
While a variety of film laminates are known in the art, a particularly useful film laminate includes a breathable barrier. Breathable barriers can comprise stretch-filled microporous film or non-filled microporous film layers. Stretch-filled microporous films are typically filled with particles or other matter and then crushed or stretched to form a fine pore network throughout the film. The pores result when the polymers in the film separate from the particle filler. This film-pore network allows gas and water vapor to pass through the film while acting as a barrier to liquids and particulate matter. The amount of filler within the film and the degree of stretching is controlled so as to create a network of micropores of a size and frequency to impart the desired level of breathability to a fabric. An exemplary stretched filled film is described in commonly assigned WO Patent Application 95/16562 to McCormack. The McCormack reference discloses a stretched filled film comprising a predominantly linear polyolefin polymer, a bonding agent and about 30 to 80% by weight calcium carbonate which can be stretched to impart breathability to the film. The stretched film may then be laminated to a nonwoven web to create a laminate that takes advantage of the strength, integrity and cloth-like aesthetics of the nonwoven web and the barrier properties of the stretched film.
Providing a low cost film lamination method that achieves both a desired level of conformance and breathability is problematic, particularly when laminating stretch-filled films. In order to achieve acceptable body conformance, the lamination method should impart good stretch (that is effective stretch with limited variability) properties, and must also be able to allow for the formation and retention of the micropores upon processing. The pores must be capable of being stretched to an extent that they remain open and yet are not enlarged to the level that would allow liquid or particulate matter to pass.
Another particularly useful laminate including a breathable barrier includes a non-filled microporous film. While a nonwoven web can be laminated to a non-filled microporous film, in order to obtain a laminate with cross-directional stretch (CD) and machine direction stretch (MD), the nonwoven web must often be separately stretched in each direction.
In order to achieve MD and CD stretch in laminates including monolithic films (that is nonporous films), the film must be separately stretched in each direction during the manufacturing process.
It has been recognized that to enhance a personal care product""s ability to conform to the body, nonwoven webs and films (and laminates thereof) which make up the personal care product are separately stretched in the CD and MD directions during manufacture, to create breathable low density/low basis weight fabric with stretch in multiple directions. A technique which has been used to stretch nonwoven webs and films in multiple directions, includes passing these sheet materials through multiple sets of intermeshing rolls. In this regard, a sheet of nonwoven web or film is fed through a series of nips formed between multiple sets of intermeshing rolls. The rolls typically have intermeshing peaks and troughs along their respective surfaces or are made of removable intermeshing disks stacked laterally along the length of axles. As the nonwoven web or film is coursed between the sets of rotating rolls, the rolls are brought together in a mating fashion, thereby forcing the surfaces of the rolls to intermesh. As a result, the material is clamped by multiple incremental nips formed between the intermeshing regions of the two rolls. The rotation of one or both of the intermeshing rolls (in opposing directions) pulls the material through the set of rolls.
As the nonwoven web or film travels between the intermeshed areas on the rolls, it is stretched by the increments between the peaks (or disks, as the case may be). This incremental stretching, as shown in FIG. 1, produces zones of variable stretch on the nonwoven web or film sheet. A machine direction, partial cross-sectional view of a nonwoven web 20 being coursed through prior art intermeshed rolls is shown in FIG. 1. The nonwoven web 20 has discrete stretch points 21 between unstretched regions 22. The stretch points 21 occur at the extended points 23 on the peaks of the rolls"" surfaces. The material is held taut between each of these peaks and stretched. This variable stretching often leads to weakening of the nonwoven material in these regions, and ultimately to material failure. In many instances, the surface of the rolls act as slitters if they are operated at high speed, with the edges of the peaks or disks piercing the nonwoven material.
Rolls with intermeshing surface troughs perpendicular to the rolls"" longitudinal axis, or that are made of stacked disks, will stretch nonwoven material in the CD direction. Rolls with intermeshing surface troughs that are parallel to the rolls"" longitudinal axis will stretch this material in the MD direction. In order to stretch biaxially, that is in both the CD and MD directions, the nonwoven material must be coursed through consecutive pairs of rolls with a first pair of rolls having troughs perpendicular to those of the second set. Therefore, biaxial stretching has heretofore required an extended production line.
In using intermeshed rolls to stretch microporous film, the film pores may not sufficiently be opened to maintain their porosity through a later lamination step. In elastic films, the pores often collapse. As a result, it is often difficult to obtain adequate stretching or pore formation over the entire width of a film sheet without rupturing the sheet. If less variability in stretch is desired to achieve high breathability, the film sheet must be run through sets of rolls multiple times. Even with repetitive rolling however, such rolls fail to allow for targeted stretching in discrete regions on the film material.
Other methods for stretching an impregnated film to produce a microporous product include the concomitant controlling of the velocity of the film introduction into a nip of troughed rolls to substantially the identical velocity of the surface velocity of the rolls. This technique, disclosed in U.S. Pat. No. 4,153,751 to Schwarz for the creation of microporous film, is best accomplished by numerous runs to achieve a desired draw. If only one pass of film is run, it is likely that the pores formed around the film filler will be oblong in shape. As a result, such pores may collapse during a later lamination step.
In order to create a nonwoven web with enhanced stretch, softness and drapability, and with lower basis weight, the shapes and sizes of the troughs on intermeshing rolls have been varied. For instance, roll peaks and troughs have been toothlike in shape, as exemplified by U.S. Pat. No. 4,153,664 to Sabee, or sinusoidal, as described in U.S. Pat. No. 5,028,289 to Rasmussen. While use of such intermeshing rolls produces a fabric having a pattern of spaced apart undrawn shaped islands in the fibers of the sheet material, the fabric stretch varies greatly by regions on the sheet. Fibers of such webs contain thick and thin zones. Therefore, while varying the shape and incline of the peaks may help in lengthening the areas of stretch on the nonwoven web material, the overall stretching varies by region.
In an effort to create CD stretching with more uniform incremental stretching, the sinusoidal peaks and troughs of rolls have been manufactured of a deformable material such as rubber. U.S. Pat. No. 4,806,300 to Walton et al. describes intermeshing rolls with a sinusoidal cross-sectional surface profile formed from circumferential rubber peaks and troughs at the surface of the rolls. However, the stretching at the incremental nips between such rolls also varies by zone. It is likely that at high speeds, the rubber peaks of such rolls would disintegrate from frictional forces. In order to achieve stretching in both the machine and cross-machine direction with such rolls, it is also necessary to run material through a series of such roll sets.
Other methods for stretching a nonwoven sheet material include the use of clamp systems such as tenter frames. However, clamp systems pose efficiency and practical limitations which make their use on an extended, high volume production line difficult.
In an effort to reduce the deformation of a nonwoven web that occurs during lamination, the non-engraved anvil roll of a thermal point bonding roll set (as opposed to the patterned roll) has been covered with rubber, such as a rubber sleeve, in order to create a larger area over which a constant nipping force is distributed. As disclosed in WO 98/28134 to Jones et al., the modified anvil roll does not provide for zoned stretching or zoned breathability of a nonwoven material, or biaxial stretching of a nonwoven material.
Therefore, despite the developments in roll technology, there exists a need for an apparatus and method for stretching a film and nonwoven web material in which the material is stretched with less variability across a nip, ie. incremental stretching is minimized. There exists a need for an apparatus and method for stretching such material in both the CD and MD direction using one set of rolls. A need also exists for a method which provides discrete areas of CD and MD stretch. There also exists a need for a method for producing CD and MD stretched materials which can be accomplished with few if any repetitions in a production line. There also exists a need for a method for producing CD and MD stretched materials using a variety of materials, including fragile web materials. Further, there exists a need for nonwoven materials which exhibit enhanced levels of stretch in the CD direction, increased breathability, and reduced basis weight. Finally, there exists a need for stretched microporous film which can be stretched in both the CD and MD directions so as to allow for pore formation in multiple directions, using one set of rolls. It is to the provision of such apparatus, method, and materials that the present invention is directed.
The present invention recognizes and addresses the foregoing disadvantages, and others of prior art constructions and methods.
Accordingly, it is an object of the present invention to provide a method for stretching non-woven webs, film, and constructions made therefrom in the cross-machine direction.
It is another object of the present invention to provide a method and apparatus for biaxially stretching nonwoven webs, film, and constructions made therefrom using one set of stretch surfaces.
It is still a further object of the present invention to provide a method for targeted stretching of nonwoven webs, film, and constructions made therefrom, in discrete zones.
It is another object of the present invention to provide stretched-filled microporous film with increased breathability in a single operation.
It is still another object of the present invention to provide nonwoven webs, film, and constructions made therefrom with cross-machine directional stretch, which materials can then be incorporated as a component of a laminate or end product.
In accordance with the present invention, the foregoing and other objects and advantages are accomplished by coursing an extensible polymeric sheet material through a deformable nip, the nip being formed between two stretch surfaces having intermeshing peaks and troughs, the surfaces of the peaks and troughs being filled and covered with a deformable material forming deformable contacting surfaces, such that as the stretch surfaces are brought together in a mating fashion to form a nip, the faces of the stretch surfaces are initially smooth and the nip is initially generally linear. Compression of the stretch surfaces leads to expansion of the nip in the MD and CD directions. In one embodiment the stretch surfaces are a pair of pressed rolls including radial peaks and troughs in the machine direction, in which the troughs and peaks are filled and covered with a low durometer rubber. In another embodiment the peaks and troughs are covered and filled with a foam. In still another embodiment, the troughs are filled with foam inserts and the peaks and troughs are further covered with a foam sheet.
In a further aspect, the invention resides in a method for stretching extensible polymeric sheet material in the cross-machine direction, comprising coursing polymeric sheet material through a deformable nip, the nip being formed between two stretch surfaces having intermeshing peaks and troughs, such that the peaks and troughs of the first of the stretch surfaces are covered and filled with a deformable material whereby the peak tips and trough valleys have less deformable material covering than the remaining peak and trough surfaces, and such that the peaks and troughs of the second stretch surface are completely covered and filled with a deformable material.
In a further aspect, the invention resides in a method for stretching extensible polymeric sheet material in the cross-machine direction, comprising coursing the sheet material through a deformable nip, the nip being formed between two stretch surfaces having intermeshing peaks and troughs, the surfaces of the peaks and troughs of one of the surfaces being filled and covered with a deformable material.
In still a further aspect, the invention resides in a nonwoven web or film material and laminates made therefrom having stretch in the cross-machine direction and breathability.